1. Technical Field of the Invention
This invention relates to a light guide pipe useful for example in a planar light source unit. The invention also relates to a planar light source unit generally, and to a liquid crystal display device.
2. Discussion of the Background
Liquid crystal display devices are broadly used in personal-computer monitors and flat-panel TV displays. A transmissive liquid crystal display device usually has a planar light source unit (backlight) arranged on a backside of the liquid crystal unit. The planar light source unit converts into planar light the light given from a linear light source such as a cold cathode fluorescent lamp or from a light source arranged with a plurality of point light source, such as an LED array.
For example, there are descriptions of planar light source units using a side-light scheme in Japanese Patent Laid-open No. 99187/1986 and Japanese Patent Laid-open No. 62104/1988 which disclose an increase in the density of a light extracting mechanism (light extractor) according to an increase in the distance from light source. The side-light scheme is a scheme having a linear light source provided at a side surface of a transparent light guide pipe of acrylic resin to convert the light from the linear light source into planar light.
Japanese Patent Laid-open No. 17/1990 and Japanese Patent Laid-open No. 84618/1990 describe a planar light source unit comprising a light guide pipe (first element) having a light incident surface, one light emitting surface orthogonal to the light incident surface and a reflecting surface opposed to the light emitting surface and formed with a light extracting mechanism (in this application another name for a light extracting mechanism is light extractor, neither of which are means-plus-function terms), light sources provided at opposite side ends of the light guide pipe, a light control sheet (second element) having a prism array provided in proximity to the light emitting surface of the light guide pipe and having triangular prisms and arranged such that the prisms have a top vertex directed toward the light emitting surface of the light guide pipe where a generating line of an arbitrary prism constituting the prisms is arranged nearly in parallel with the light source, and a reflecting sheet (reflecting surface) is provided in proximity to the light reflecting surface of the light guide pipe. The latter reference (JP Laid-open No. 84618/1990) also discloses a uniform surface-roughening treatment of the light emitting surface of the first element. The inventions described in both publications emit light in a particular direction. These, however, are unsatisfactory with respect to practical optical characteristics, e.g., satisfactory evenness is not achieved in the emission light on the light-emitting surface (see Japanese Patent Laid-open No. 18879/1998). In particular, no existing planar light source device adequately provides a broad view angle suited for a planar light source unit used as a liquid crystal display for monitors or thin panel televisions.
Japanese Patent Laid-open No. 18879/1994 describes a planar light source unit comprising a light guide pipe having a light incident surface, one light emitting surface orthogonal to the light incident surface and a light reflecting surface opposed to the light emitting surface and formed with a light extracting mechanism, the light guide pipe having a roughened surface having a directivity emission function or a plurality of lens units provided in one or both of the light guide pipe light emitting surface and reflecting surface, a smooth area formed in a surface having the roughened surface or the lens units so that the ratio of the smooth area increases as the light emitting surface is approached thereby having a control function to make the luminance value of the light emitted from the light emitting planar light sources provided at opposite side ends of the light guide pipe even, a light control sheet provided in proximity to the light emitting surface of the light guide pipe and having a prism array comprising triangular prisms so that the triangular prisms have a top vertex directed toward the light emitting surface of the light guide pipe and a generating line of an arbitrary prism constituting the prism array arranged nearly in parallel with the light source, a reflecting sheet provided in proximity to the reflecting surface of the light guide pipe. According to this invention, front brightness is nearly sufficiently achieved. However, the view angle characteristic is extremely narrow in a direction perpendicular to a major axis of the light source and hence not well suited in application to a liquid crystal display device or the like. Even with this reference, it is still difficult to provide brightness evenly throughout the entire light guide pipe surface in attempting to solve the problem of the existing dark region in a light guide pipe surface, as shown in FIG. 13 and FIG. 14 herein. (The reason a dark region 22 exists is explained below based on FIG. 14.) A fluorescent lamp, such as a cold cathode fluorescent lamp, has a light emission characteristic similar to a light emission angular distribution as seen in so-called Lambert type scattering with high light emission without variation in each of the light emission angles. Consideration is made on the light emitted from a linear light source taking into account such a light distribution characteristic. In FIG. 14 the light emitted, e.g. from a major portion of each linear light source propagates up to a center region in a light emitting surface of a light guide pipe. That is, most of the linear light source contributes to light emission through the center region. On the other hand, in the dark region 22 of the light-emitting surface (FIG. 14), there is no effective reach of the light from the part of the light sources positioned far from the dark region (e.g. emission light 34c in the dark region 37. Due to this, the amount of light reaching the part corresponding to the dark regions 31, 32 of the light-reflecting surface is insufficient.
Meanwhile, the light rays propagating into the light guide pipe at a middle area of two light sources (FIG. 14) or its vicinity has directivity distributed nearly symmetric left and right as represented by optical vectors of emitting light as viewed from above the light emitting surface. However, the directivity of the light ray in respective areas near the light sources distributes asymmetric left and right as represented by optical vectors of emitting light.
A light guide pipe using a conventional light extracting mechanism (extractor), which means a print-type light guide pipe made by a normal screen printing process using light scattering ink, such as resins composed of TiO2 or SiO2 microparticles, has directivity varying so much because of the multi-scattering process with microparticles, thus, this asymmetric directivity caused no problems. However, a light guide pipe which has surface-roughened protrusions as the light extracting mechanism has directivity which does not vary so much when light diffuses-reflects through the light extracting mechanism. Asymmetric left and right directivity distribution as represented by the optical vectors of emitting light is maintained as it is, and light emits from the light guide pipe. Due to this, at the side area of the light guide pipe, in the vicinity of the light sources, the brightness of light is insufficient in the most-necessary forward direction.
A conventional planar light source unit is also set to have a maximum brightness in a normal line direction from the light emitting surface. A two-lamp type planar light source unit is set to provide a maximum brightness in a normal line direction of the light-emitting surface for light from each of the light sources. The view angle characteristic in this case is as shown in FIG. 23. That is, in FIG. 23, the dotted lines represent a brightness distribution for each one light source, while solid line is a combination of light from the two light sources. As understood from FIG. 23, the conventional planar light source unit has a maximum brightness at a view angle of nearly 90 degrees for a combination of emission light from the two light sources. However, when used in a large-sized liquid crystal display, desirable optical characteristics are not available due to flickering or the like on the screen.
In the conventional art, there is a problem that it is impossible to obtain an illuminating optical system having an even brightness characteristic over the entire light guide pipe surface, i.e. a dark region exists in the light guide pipe surface and a bright line might occur in the vicinity of the light incident surface of the light guide pipe, especially in a light guide pipe which has surface-roughened protrusions as a light extracting mechanism (light extractor).
In conventional illuminating optical systems, there is also a problem in that a viewing angle characteristic cannot be kept sufficiently broad when the optical system uses a prism array sheet comprising triangular prisms and the triangular prisms have a top vertex directed toward the light emitting surface of the light guide pipe.
Also in the conventional planar light source unit, there is another problem in that it is difficult to obtain a planar light source which is simple in structure and easy to manufacture.
JP U5-69782, incorporated herein by reference, discloses a system of roughened protrusions and/or concaves limited in shape to circles, squares and ellipses. This reference does not disclose protrusions or concaves all having their long axis oriented in a common way. In this sense, the present invention can utilize the technology, elements, etc. of this reference while improving performance by orienting the long axis of the protrusions and/or concaves.
It is one object of the present invention to provide a planar light source unit solving at least one of the above-mentioned problems and a liquid crystal display device using such a planar light source unit(see FIG. 1).
Accordingly, a first embodiment or form of the present invention is a light guide pipe both by itself, and contained within a planar light source unit. Briefly, the planar light source unit comprises: the invention light guide pipe 2 having a light incident surface 3, one light emitting surface 4 orthogonal to the light incident surface 3, a reflecting surface 5 opposed to the light emitting surface, and a light extracting mechanism (light extractor) 7 formed on at least one of the light emitting surface and the reflecting surface;
Linear light sources 8 arranged at opposite side ends of the light guide pipe; a light control sheet 11 provided in proximity to the light emitting surface of the light guide pipe and having a prism array 12 optionally comprising triangular prisms such that the triangular prisms have a top vertex directed toward the light emitting surface of the light guide pipe and an arbitrary prism constituting a line of the triangular prisms, vertices of the prism array has a generating line positioned generally (5 micrometer to 200 micrometer, more preferably 10 micrometer to 150 micrometer, the most preferably 20 micrometer to 100 micrometer) in parallel with the light incident surface (see, e.g., FIG. 1 where the prism vertices form lines in parallel with the light incident surface); and a reflecting sheet 14 provided close to the reflecting surface of the light guide pipe, namely the distance between a reflecting sheet and the light guide pipe are within 500 micrometer, more preferably 400 micrometer, the most preferably 250 micrometers (including in contact with); wherein the light extractor comprises a plurality of surface-roughened protrusions and/or concaves provided on at least one surface of the light guide pipe and the surface-roughened protrusions and/or concaves all have a major axis in a direction nearly perpendicular to the long axis of the linear light source. For example, the major axis has the angle 60 to 120 degree, more preferably 70 to 110 degree, the most preferably 80 to 100 degree to the long axis of linear light source. With this planar light source unit it is possible to effectively extract the light from the light source and increase the brightness for the entire planar light source unit. (For the light guide pipe itself, it is preferable that the protrusions and/or concaves all have their individual major axis within 30 degrees of all others and used not be specially oriented with regard to an external reference. However, precise specification for desirable orientation for these light extractors are defined by the parameter of effective aspect ratio described below.
A second form of the invention is a planar light source unit where a scatter enhancing region is provided in at least one side triangular portion of the reflecting surface and/or the light emitting surface of the light guide pipe, said triangular portion having as a base a different side of said light guide pipe from those opposite ends provided with the light sources, the scatter enhancing region having a higher density of surface-roughened protrusions and/or concaves as compared to other portions of the reflecting surface and light emitting surface at the same distance from the light source. In conventional planar light source units, there exists a dark region as stated before. The provision of a scatter enhancing region having a higher relative density of protrusions and/or concaves in region corresponding to this dark region makes it possible to effectively introduce light to the dark region and hence provide even brightness in the planar light source unit.
A third form of the invention is a planar light source unit, comprising: a light guide pipe 2 having a light incident surface 3, one light emitting surface orthogonal to the light incident surface, a reflecting surface 5 opposed to the light emitting surface and a light extracting mechanism (light extractor) 7 formed on at least one of the light emitting surface and the reflecting surface; two light sources arranged at opposite side ends of the light guide pipe; a light control sheet provided in proximity to the light emitting surface of the light guide pipe, for example, a light control sheet exists within 500 micrometer, more preferably 400 micrometer, the most preferably 250 micrometer to the light guide pipe (including in contact with), and a reflecting sheet provided close to the reflecting surface of the light guide pipe, for example, the reflecting sheet exists within 500 micrometer, more preferably 400 micrometer, the most preferably 250 micrometer to the light guide pipe (including contact); arranged such that when any one of the light sources is put on, a vertical (i.e. orthogonal to the light incident surface) view angle distribution characteristic of an emission light ray observed immediately above the light control sheet in a normal line direction of the light emitting surface is such that when represented on a diagram having luminance [cd/m2] taken on a vertical axis and an emitting angle [degrees] of an emission light ray on a horizontal axis, a line representing a vertical view angle distribution characteristic of an emission light ray in a normal direction of the light emitting surface has steep and slant regions, in other words, above mentioned curve usually has mountain shape, and the steep region means the one side of that curve with a peak as a boundary that have smaller half height width at half luminance intensity from peak luminance intensity. And the slant region means another side of that curve. with a peak as a boundary; and where the two light sources are put on, lines representing view angle distribution characteristics given respectively by said light sources intersect at their slant regions.
In the case of combining the light as represented in FIG. 21, where the lines representing view angle distribution characteristics given respectively by the light sources intersect at their steep region, a combined distribution of emission light assumes a two-mountain-formed distribution. This is not necessarily preferred. The absolute and relative slopes of the curves (lines) in the Figures provide preferred steep and slant regions.